Enhanced Electrocatalytic Properties of Transition-Metal

Mar 25, 2013 - The lithium intercalation–exfoliation process makes the basal planes of chemically exfoliated MoS2 and WS2 sheets much more defective...
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Enhanced Electrocatalytic Properties of Transition-Metal Dichalcogenides Sheets by Spontaneous Gold Nanoparticle Decoration Jaemyung Kim,† Segi Byun,†,‡ Alexander J. Smith,† Jin Yu,‡ and Jiaxing Huang*,† †

Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea

‡

S Supporting Information *

ABSTRACT: Here, we report that transition-metal dichalcogenides such as MoS2 and WS2 can be decorated with gold nanoparticles by a spontaneous redox reaction with hexachloroauric acid in water. The resulting gold nanoparticles tend to grow at defective sites, and therefore, selective decorations at the edges and the line defects in the basal planes of bulk single crystals were observed. The lithium intercalation−exfoliation process makes the basal planes of chemically exfoliated MoS2 and WS2 sheets much more defective than their single-crystalline counterparts, leading to a more uniform and higher-density deposition of gold nanoparticles. Due to the greatly improved charge transport between adjacent sheets, the resulting MoS2/Au and WS2/Au hybrids show significantly enhanced electrocatalytic performance toward hydrogen evolution reactions.

SECTION: Energy Conversion and Storage; Energy and Charge Transport

T

synthesized sheets because the highly strained 1T phases existing on the single layers could help to lower the overpotential needed for HERs.23 A challenge with using the chemically exfoliated TMDC sheets is their tendency to restack during materials processing, which will not only hinder the vertical charge transport but also limit the accessibility of protons to the catalytically active sites of TMDC. These problems may be addressed by decorating the sheets with electrically conductive metal nanoparticles, such as Au, which could enhance the charge transport along interplanar directions and act as spacers to inhibit restacking. The use of high-cost materials such as gold can be justified if the amount needed is small compared to the TMDC. Herein, we report spontaneous decoration of Au nanoparticles on MoS2 and WS2 sheets upon direct reaction with a gold precursor hexachloroauric acid (HAuCl4) in water. The decoration reaction occurs on the surface of both bulk TMDC single crystals and chemically exfoliated sheets. The presence of Au nanoparticles on chemically exfoliated MoS2 and WS2 nanosheets greatly improves charge transport between the sheets and enhances their HER catalytic efficiency. When a piece of single-crystalline MoS2 is immersed into an aqueous solution of HAuCl4 (typical concentration of 1 × 10−3

wo-dimensional (2D) sheets exfoliated from bulk layered inorganic compounds, such as transition-metal dichalcogenides (TMDC) MoS2 and WS2, have gained wide attention in recent years as graphene analogues with interesting electronic, optical, mechanical, and catalytic properties.1 Although significant efforts have been made on the investigation of the electronic and optical properties of singleor few-layered TMDCs,1−3 interest in the chemical properties and surface functionalization of these 2D sheets has only recently started to emerge. As has been extensively shown with graphene-based materials, decorating the sheets with nanoparticles is an effective way to functionalize the surface and render new and enhanced properties for areas such as energy storage and catalysis.4−11 Several strategies have been demonstrated to decorate TMDC with metal nanoparticles.12−17 Typically, this has been done by either simply mixing the individual constituents12,13 or adding the TMDC sheets in the reaction between the metal precursors and reducing agents, depositing the nanoparticles in situ onto the sheets.15,16 Because TMDC compounds have rich redox chemistry, however, they themselves could directly react with metal precursors to allow a very straightforward and clean synthesis of metal-decorated sheets. TMDC compounds, especially MoS2 and WS2, are emerging electrocatalysts for hydrogen evolution reactions (HERs).18−21 Recently, HERs catalyzed by chemically exfoliated TMDC layers were reported.22,23 Significantly, evidence suggests that chemically exfoliated TMDC sheets have advantages over © 2013 American Chemical Society

Received: March 6, 2013 Accepted: March 25, 2013 Published: March 25, 2013 1227

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would expect such exfoliated sheets to become much more defective and reactive. Indeed, we noticed that some sheets produced by chemical exfoliation appear more defective even at the microscopic scale when observed under an atomic force microscope (AFM). Figure S1 (Supporting Information) shows AFM images contrasting the MoS2 sheets obtained by chemical exfoliation and mechanical exfoliation from a single crystal. The chemically exfoliated sheets have jagged edges and sometimes even cracks in the basal plane, while the single layer produced by mechanical exfoliation shows sharp edges and a smooth surface. Therefore, we expect that chemically exfoliated MoS2 sheets will have many more reactive sites, even in the basal planes, for gold decoration. Figure 2a shows UV/vis absorption spectra of an aqueous dispersion of chemically exfoliated MoS2 sheets, an aqueous

M), spontaneous reaction can occur, decorating the MoS2 crystal with gold nanoparticles, as shown in the scanning electron microscopy (SEM) images shown in Figure 1.

Figure 1. SEM images showing gold nanoparticles preferentially grown at defect sites such as (a) edges or (b) line defects on the basal plane of a MoS2 single crystal after immersion in an aqueous solution of HAuCl4. The inset in (a) shows a higher magnification image. (c) An energy diagram showing that the Fermi level of MoS2 lies above the reduction potential of Au3+ (+1.002 V versus SHE). Therefore, a spontaneous redox reaction should occur between MoS2 and HAuCl4.

Notably, we found that the resulting gold nanoparticles preferentially decorate defect sites of MoS2 single crystals, such as edges (Figure 1a) or line defects on the basal plane (Figure 1b), while the main basal plane remains largely unaffected on the microscopic scale. Previously, gold deposited by thermal evaporation was observed to preferentially nucleate at the defect sites of bulk MoS2.24 Spontaneous reduction of gold ions by carbon-based materials such as carbon nanotubes or graphene has also been observed.25−27 Here, the underlying mechanism is also likely due to the redox reaction between MoS2 and gold ions. The work function of MoS2 is 5.2 eV,28 situating the Fermi level of MoS2 well above the reduction potential of AuCl4− (+1.002 V versus the standard hydrogen electrode (SHE), Figure 1c).29 Therefore, MoS2/AuCl4− should form a redox pair, allowing spontaneous electron transfer from MoS2 to gold ions to occur and leading to the formation of gold nanoparticles. It follows that these gold nanoparticles would preferentially nucleate at the highly energetic defect sites such as edges or line defects. One may also expect a small portion of Mo atoms to be oxidized to water-soluble, higher valence forms and leached into the solution. We found that the same reaction that occurs with bulk MoS2 can occur with chemically exfoliated MoS2 sheets. To prepare single layers of MoS2, we employed the lithium intercalation− exfoliation approach developed by Morrison and co-workers.30 After extensive purification, a stable colloidal dispersion of MoS2 single-layered sheets dispersed in water can be obtained. The violent intercalation−exfoliation reactions produce high local strain, triggering a partial phase transformation from trigonal prismatic (2H) to octahedral (1T).31,32 Therefore, one

Figure 2. The rapid spontaneous reaction between chemically exfoliated colloidal MoS2 and HAuCl4 produces Au-decorated sheets. (a) UV/vis spectra of a MoS2 dispersion (orange squares), HAuCl4 solution (blue diamonds), and their mixture (magenta circles). Upon mixing, the HAuCl4 peak diminishes, as indicated by the blue arrow, and a new absorption band at around 550 nm corresponding to Au nanoparticles emerges, as indicated by the black arrow. The photo in the inset shows the colors of MoS2, HAuCl4, and their mixture. (b) TEM images showing a MoS2 sheet uniformly decorated by Au nanoparticles (top). On some sheets, Au nanoparticles can be seen preferentially nucleating along the cracks (bottom). (c) The XRD pattern of the product has strong diffraction peaks matching the standard diffraction pattern of gold (JCPDS 04-0784). (d) No significant Mo3d band shifts can be detected by XPS before (top) and after (bottom) gold decoration, suggesting that most of MoS2 remains unoxidized upon the reaction.

solution of HAuCl4, and a mixture of the two. When a small aliquot of HAuCl4 was added into chemically exfoliated MoS2, the absorption peak of the gold precursor at around 215 nm decreased significantly, while a new absorption peak corresponding to the Au plasmon band at around 550 nm emerged, suggesting the consumption of Au3+ and the formation of gold nanoparticles. Upon mixing, an obvious color change was observed as the dispersion changed from yellow to purple 1228

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(Figure 2a, inset). The reaction occurred instantaneously and completed within seconds. Transmission electron microscopy (TEM) images of the chemically exfoliated MoS2 sheets after the gold decoration show that unlike in the case of a single crystal (Figure 1), gold nanoparticles uniformly decorated both the edges and the basal planes of the MoS2 single layers (Figure 2b, top). In some cases, the gold nanoparticles can be seen clearly following cracks in the basal plane of the sheets (Figure 2b, bottom), further suggesting that the gold nucleation preferentially occurs at defect sites. Figure S2 (Supporting Information) shows high-resolution TEM (HRTEM) images of gold nanoparticles on MoS2. The particles do not appear to be single-crystalline, similar to what was observed in a recent report.17 X-ray diffraction (XRD) (Figure 2c) and X-ray photoelectron spectroscopy (XPS) studies (Figure S3, Supporting Information) confirmed that the gold precursor was indeed reduced. XPS data exhibit no significant shift in Mo3d bands before and after gold decoration (Figure 2d). A Raman spectrum obtained from a sample drop-casted on a Si substrate showed strong E12g and A1g bands of MoS2 at 383 and 408 cm−1, respectively (Figure S4, Supporting Information), suggesting that most of MoS2 in the hybrid material remains unoxidized. However, when a significant molar fraction of gold was incorporated into MoS2 (>60 mol %), a more significant contribution from Mo6+ 3d3/2 at around 236 eV and Mo6+ 3d5/2 at around 232 eV could be observed (Figure S5, Supporting Information) in XPS spectra, suggesting the oxidation of MoS2 to molybdic acid. The density and size of the gold nanoparticles decorating the MoS2 sheets can be easily tuned by controlling the amount of HAuCl4 added into the dispersion. For instance, we observed a red shift in the UV/vis spectra of MoS2/Au hybrids by gradually increasing the molar concentration of HAuCl4 added into the MoS2 dispersions of a fixed concentration (Figure S6, Supporting Information), indicating the formation of larger gold nanoparticles. This was also confirmed by the TEM images shown in Figure S7 (Supporting Information). The extent of gold ion reduction by the spontaneous redox reaction can be estimated by monitoring the UV/vis absorption peak of Au3+ at around 215 nm before and after the reaction, which can be used to quantify the loading level of Au in the final MoS2/Au hybrid. TMDCs, most notably MoS2 and WS2, have recently gained much interest as promising electrocatalysts for hydrogen evolution.18,19,33,34 Their catalytic efficiency, however, is limited by inherently low interparticle conductivity between adjacent van der Waals bonded sheets.18 Au decoration of MoS2 sheets should therefore greatly improve the charge transport between neighboring sheets, leading to enhanced electrocatalytic performances. Previous density functional theory studies suggested that only one-quarter of the edge sites contribute to the H2 evolution at a given time.35,36 Because the reduced Au atoms segregated into nanoparticles on the edge, Au decoration is unlikely to cause significant blockage of the active edge sites, especially at a low loading level. HER measurements were therefore undertaken with a standard three-electrode setup in 0.5 M H 2 SO 4 (see Experimental Methods) to assess the influence of the Au nanoparticles on MoS2’s electrocatalytic behavior. Figure 3a shows polarization curves of the MoS2/Au hybrids with increasing molar loading levels of gold. As the amount of gold nanoparticles increases, the overpotential (η) decreases, while the current density (j) increases, exhibiting higher HER

Figure 3. Electrocatalytic HER performance of MoS2/Au. (a) As the loading level of gold increases, the overpotential decreases, while the current density increases, likely due to the enhanced charge transport in the interplanar direction (b), as suggested by greatly decreased charge-transport impedance (Z′).

electrocatalytic performance compared to that of the bare chemically exfoliated MoS2 sheets. The overpotential values of bare MoS2 reported here are much lower compared to previously reported values for chemically exfoliated MoS2 sheets.22 The Tafel slopes (b) estimated from the linear portions of Tafel plots (not shown) also gradually decreased from 65.53 to 56.97 mV/dec as the molar percentages of gold incorporated into MoS2 were increased from 0 to 39.5%, respectively, leading to the higher current densities at the same overpotential with gold decoration. Impedance measurements performed on the same samples (Figure 3b) show that the charge-transport impedance (Z′) was significantly reduced by incorporating even a very small amount (3.3 mol %) of gold, suggesting that the observed HER performance enhancement originates from improved charge transport through the gold nanoparticles. The overpotentials, current densities, Tafel slopes, and charge-transport impedances of MoS2 with varying amounts of gold incorporation are summarized in Table 1. Table 1. Summary of HER Performance Parameters of MoS2 Sheets Decorated with Varying Amounts of Gold Nanoparticles

a

Au (mol %)

η (V)

jη=0.4 Va (mA/cm2)

b (mV/dec)

Z′ (Ω)

0 2.0 6.7 24.6 39.5

0.252 0.242 0.223 0.219 0.205

6.08 9.08 14.14 16.99 22.62

65.53 64.53 58.97 56.98 56.97

738 623 315 207 162

Current densities at η = 0.4 V.

Because WS2 exhibits very similar electronic, optical, and chemical properties to MoS2, we found that the gold decoration chemistry also works with WS2, which has a work function of 5.1 eV.37 Adding a small aliquot of HAuCl4 solution into an aqueous dispersion of chemically exfoliated WS2 sheets can also result in spontaneous Au decoration (Figure 4a, inset). UV/vis spectra in Figure 4a show that upon reaction, the absorption peak of Au3+ at around 215 nm significantly decreased, while the new band corresponding to Au nanoparticles at around 550 nm started to appear. A TEM image of the sample also confirmed the successful decoration of gold nanoparticles onto WS2 sheets (Figure 4b), similar to the case of MoS2 (Figure 2b). As with MoS2, Au decoration also enhances the HER activity of WS2 sheets (Figure 4c), as well as the charge1229

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Table 2. Summary of HER Performance Parameters of WS2 Sheets Decorated with Varying Amounts of Gold Nanoparticles

a

Au (mol %)

η (V)

jη=0.4 Va (mA/cm2)

b (mV/dec)

Z′ (Ω)

0 3.3 12.9 21.1 36.3

0.282 0.264 0.247 0.23 0.233

1.91 3.72 7.91 15.67 14.5

110.28 80.79 66.67 56.69 57.49

7500 1909 735 292 335

Current densities at η = 0.4 V.

acid in water. The resulting gold nanoparticles tend to grow at the defective sites of the sheets. The chemical intercalation− exfoliation process makes the basal plane of the sheets much more defective than their single-crystalline counterparts, thus resulting in a higher-density deposition of Au nanoparticles. The resulting MoS2/Au and WS2/Au hybrids show significantly enhanced electrocatalytic performance toward HER, which is attributed to greatly improved charge transport between neighboring sheets, as indicated by reduced charge-transport impedance. Au-decorated TMDC sheets could become a base material for exploring surface-plasmon-enhanced photo- and electrocatalysts for the oxidation of organic molecules or HER.39,40 With abundant chemistry available for transitionmetal compounds, chemically exfoliated TMDC sheets should be versatile platforms for constructing highly functional inorganic 2D systems.

Figure 4. Spontaneous decoration of gold nanoparticles on chemically exfoliated WS2 sheets. (a) As in the case of MoS2, HAuCl4 is consumed upon reacting with WS2 to produce gold nanoparticles, as indicated by a new absorption band at around 550 nm and the color change of the dispersion from yellow to purple (inset). (b) TEM image showing gold nanoparticles uniformly decorating a WS2 sheet. (c) Polarization curves and (d) impedance spectra showing enhanced electrocatalytic HER performance of WS2 sheets and reduced chargetransport impedance, respectively, upon gold decoration.

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EXPERIMENTAL METHODS All chemicals except for MoS2 single crystals (SPI Supplies) were purchased from Sigma-Aldrich. To make chemically exfoliated MoS2 single layers, MoS2 powder was stirred in a 1.6 M solution of n-butyllithium in hexane for 2 days in a N2-filled glovebox, followed by a reaction with water outside of the glovebox and extensive purification steps. In the case of WS2, Li was intercalated by a solvothermal method, as previously reported.41 To decorate TMDC with gold nanoparticles, a small aliquot of HAuCl4 solution in water was added into an aqueous dispersion of chemically exfoliated MoS2 or WS2 at room temperature. The reaction occurred instantaneously and completed within seconds. SEM and TEM images were taken on an FEI Nova 600 SEM and a JEOL 2100F TEM, respectively. AFM images were taken on a Park Systems XE100 under tapping mode. UV/vis absorption spectra were measured with an Agilent 8653 UV/vis spectrometer. XRD patterns were obtained on a Rigaku D/Max powder diffractometer with Cu Kα radiation, and Raman spectroscopy was performed using a WITec Alpha 300 system with an excitation wavelength of 532 nm. XPS spectra were recorded on a Thermo Scientific ESCALAB 250Xi. Electrocatalytic measurements were performed using an AUTOLAB PGASTAT 302N potentiostat with a standard three-electrode setup in 0.5 M H2SO4 (scan rate = 5 mV/s). Working electrodes were prepared by drop-casting the samples on a glassy carbon electrode, and a graphite rod and a Ag/AgCl electrode saturated with KCl were used as a counter electrode and a reference electrode, respectively. The reference electrode was calibrated with respect to reversible hydrogen electrode (RHE). Impedance measurements were carried out at an overpotential of 0.3 V from 106 to 0.01 Hz with an AC voltage of 5 mV.

transport impedance (Figure 4d). Compared to MoS2, gold decoration of WS2 resulted in an even more significant enhancement in HER activity and a reduction in chargetransport impedance, especially at low loading level. This may be attributed to the lower initial electrical conductivity of the chemically exfoliated WS2. For both MoS2 and WS2, the Li intercalation−exfoliation process induces a partial phase transition of the sheets from the semiconducting 2H phase to metallic 1T phase. Becuase it is more difficult to intercalate Li into WS2 than MoS2,38 we postulate that the Li intercalation− exfoliation was less effective for WS2 in our experiments, leading to a lower degree of 2H to 1T phase transition. This would result in a lower initial conductivity due to a smaller fraction of the more conducting 1T phase and would lead to a larger value of initial charge-transport impedance. Hence, a small degree of gold decoration (3.3 mol %) could result in over 25% improvement in electron transport of the hybrid material. Furthermore, less effective Li intercalation should result in a smaller density of catalytically reactive sites in the chemically exfoliated WS2 sheets. As a result, decreased HER efficiency of the WS2/Au hybrid can be observed at high loading level of Au (>30 mol %) (Figure S8, Supporting Information), suggesting that a significant portion of the active sites are blocked by Au. This suggests that the main contribution of Au nanoparticles is to promote the charge transport between adjacent layers. The HER performance parameters of WS2 with varying amounts of gold incorporation are summarized in Table 2. In summary, we demonstrated that MoS2 and WS2 can be decorated with gold nanoparticles by a spontaneous redox reaction between the TMDC compounds and hexachloroauric 1230

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ASSOCIATED CONTENT

S Supporting Information *

AFM images of mechanically or chemically exfoliated MoS2 sheets, XPS, Raman, and UV/vis spectra of MoS2/Au hybrids, TEM and HRTEM images of MoS 2/Au hybrids, and polarization curves of WS2/Au hybrids with excessive gold decoration. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.H. thanks the Alfred P. Sloan Research Foundation for a Fellowship, the Sony Corporation for a gift donation, and the NSF CAREER Award (DMR 0955612). J.K. gratefully acknowledges the support from the Ryan Fellowship and the Northwestern University International Institute for Nanotechnology. S.B. was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the MEST (Project No. 2012-000181). We thank I. S. Kim and Y.-K. Huang for technical assistance and NUANCE and NU-MRSEC for the use of their facilities.

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